1. Field of the Invention
The present invention relates to an insulated electric wire, such as an enamel wire, which is superior in ease of coil insertion.
2. Description of Related Art
In a motor, a transformer, and the like, a plurality of coils, each of which is formed by winding an insulated electric wire, are inserted into, e.g., stator slots and then the ends of each of two inserted coils are mutually joined by welding or another method. Since an insulated electric wire is wound at high speed to form a coil, it is necessary to reduce the possibility in which excoriations are made on the surface of the insulated electric wire during the forming of the coil. Accordingly, there is a demand for insulated electric wires that are superior in ease of winding, i.e., have a highly lubricative surface.
In a method proposed to improve the lubrication property of an insulated electric wire, e.g., resin paint formed by adding a lubricant such as polyethylene oxide to a base resin is applied to an insulated layer and baked to form a lubricating layer (insulating coating) (see, e.g., JP-A 2007-213908). In another method proposed, resin paint formed by blending a stabilized isocyanate compound and lubricant in a base resin is applied to an electric conductor and baked to form a lubricating layer (see, e.g., JP-A Hei 9 (1997)-45143). In still another method proposed, resin paint formed by blending ester titanate in a base resin is applied to an electric conductor and baked to form a lubricating layer (see, e.g., JP-A Hei 7 (1995)-134912).
Recent motors and transformers are demanded to have high efficiency to save energy. To meet this demand, coils are inserted into stator slots so that almost no clearance is left therein, so as to increase the ratio of the cross section of the conductor of the insulated electric wire to the cross section of the stator slot (the ratio is called the space ratio). Accordingly, to reduce the possibility in which excoriations are generated on the surface of the insulated electric wire when the coil is inserted, it is also demanded that the insulated electric wire be superior in ease of coil insertion, i.e., a force to insert the coil into the stator slot be reduced.
Coils formed from conventional insulated electric wires have not been adequately easily inserted. A way to solve this problem has been to increase the amount of lubricant to be added to insulating paint so that these coils are easily inserted. In this case, however, the resulting coating (lubricating layer) has become cloudy due to the excessive lubricant, or bubbles, grains or unevenness have occurred on the surface of the coating, impairing its appearance (shape).